It is known, in the prior art, to form shaped articles from non-melt fabricatable (i.e., intractable) polymeric resins using non-extrusion techniques, as it is to extrusion-form shaped articles from resins that are melt formable. Prior to this invention, however, the art had found no techniques for forming shaped articles from non-melt fabricatable, particulate aromatic polyamide or polyimide resins, using ram-extrusion techniques.
Intractable, high temperature, polymeric resins having a glass transition temperature of at least 250.degree. C. tend to degrade well below their crystalline melting points and thus cannot be fabricated in a molten state. Further, it is known that the brittleness of such resins has prevented the use of known extrusion techniques in forming shaped articles. For this reason the methods which have evolved for forming shaped articles from this specific type of resin have been primarily compressive in nature, using combinations of pressure and heat.
A typical method of preparing shaped articles from these non-melt fabricatable type polymeric resins is shown in U.S. Pat. No. 3,413,394 to Jordan, which teaches subjecting such powder to a pressure of at least 69 MPa while maintaining the temperature of the resin at between room temperature and the temperature at which the powder will coalesce under the applied pressure, releasing the pressure on the pressed powder, and heating the pressed powder to a temperature of at least 400.degree. C. in the substantial absence of applied pressure for from about 5 to about 20 minutes to coalesce such powder into the shaped article. This method is non-continuous and the mold in which the powder is pressed must be recharged each time an article is prepared. Additionally, the ability to make finely dimensioned shaped articles such a long rods and tubes having improved tensile strength and other desired properties is limited when using this method.
Another compressive-type method in wide use involves charging polyimide resin to rubber bags and then subjecting the rubber bags to approximately 69 MPa pressure at room temperature in an autoclave. After removing the rubber bags, the green shaped articles or preforms are transferred to a hot pressure vessel where they are subjected beneath a molten lead bath to a temperature of 420.degree. C. at 103 MPa pressure. After removal from the hot pressure vessel the preform is subjected to an acid treatment to remove the lead coating. The preform at this point is characteristically quite irregular dimensionally and in many cases "bowed" as well and requires either machining or centerless grinding to provide a finished article suitable for automatic machine work. All in all, this method is characterized by low productivity, low yield, and high cost.
Ram extrusion of materials characterized by their high ductility, such as polytetrafluoroethylene resins, and metals, such as copper and aluminum, through a die is well known in the art. It has long been known that this type method, being continuous in nature and having the capability of imparting orientation to the shaped article, might also well prove best for non-melt fabricatable polymeric resins if problems, such as cracking of the resin during extrusion, could be solved.
An example of a typical extrusion method is known in U.S. Pat. No. 2,863,174 to Schuman et al. In using known dies of this type for forming articles from non-melt fabricatable polyimide and polyamide resins, however, cracking occurs. Since the art could not solve this cracking problem during extrusion, and since no methods were known for extruding non-melt fabricatable resins, other less desirable batch methods involving pressure/temperature combinations, as described above, were developed.
This invention has solved this problem by designing a new die having a configuration which enables non-melt fabricatable resins to be extruded without cracking, and which imparts in the shaped article so extruded improved properties heretofore not known to the art.
No teaching of the solid state extrusion of a non-melt fabricatable material, such as an aromatic polyimide resin, was known, prior to the discovery of this die and of the method of this invention. Such new method utilizing the direct ram extrusion of aromatic polyimide and aromatic polyamide resins to form shaped articles eliminates the difficulties, high cost, and low yield characteristics of the current compressive processes. In view of the intractability and brittleness of green, or unsintered, polyimide and polyamide moldings, it was surprising to find that high quality articles could be made by such an extrusion method.
The die profile is a cardinal parameter in this extrusion method; it must provide back pressure through a reduction zone as well as controlled relief in the exit area of the die.
Relative to the current compressive processes for producing shaped articles, this extrusion process offers (1) a marked reduction in the cost of manufacture, (2) a dimensionally precise product, (3) a wide range of shapes, and (4) a marked advantage in producing small diameter stock shapes. Beyond this, operating difficulties associated with the compressive processes, such as lead penetration, bending of the stock shapes due to the buoyant effects of the molten lead, and the gross dimensional variability across the axial length of the stock with the attendant yield loss in finishing are eliminated.
The above-mentioned patents do not show, nor does any known art show, an extrusion method for forming a shaped article from non-melt fabricatable polymeric material. Instead the art, knowing that the brittleness of the resin causes cracking of article during extrusion, turned to compressive techniques to solve this problem.
Accordingly, prior to this invention, there was a need for an extrusion method that will work in continuously forming shaped articles from high temperature, aromatic polyamide and polyimide resins, to improve both the properties of the shaped article and the method itself. This invention, in solving the problems previously mentioned, provides such a method and, in so doing, gives to the extrusion and shaped article arts improvements heretofore not known to them.